Banded multiple element fuse



March 16, 1954 Q G DEWEY BANDED MULTIPLE ELEMENT FUSE Filed March 20,1953 a q x m n w A? t Ve A mud INu/ H c y b r IAN IIIIA Length of fusePatented Mar. 16, 1954 BANDED MULTIPLE "ELEMENT FUSE Clyde G. Dewey,-Philadelphia,.Pa;, assignur to.-

General Electric Company, .a ..corporation xof Application March-20,1953, SeriaI-No.343,718

(Cl. 200-121)- a '3 Claims. 1 The invention relates to fuse and more:particularly to power fuse units of the currentzlimiting type having aplurality of fusibleelements operating in parallel current interrupting.re-

lation.

Suchpower fuse units are extensivelyused in .lieu. of the relativelymore-expensive andlarger sizecircuit breakers forprotecting relativelyhigh voltage power circuits upon the occurrence of faults, particularlyshort circuits. In such short circuit protective service, themulti-element current limiting type of fuse has the outstandingadvantage over circuit breakers of initially limiting the excesscurrentto a value which .is only a fraction of that which the power circuitis.1

otherwise capable of supplying.

To obtain such current limiting action, the multiple fuse: elementsordinarily are in therform of relatively long fine Wires ofareadilyvaporizable metalsuch as silver or zinc andv embedded inintimate heat. exchange relation throughout their activelength in aninert, granular refractory-arc extinguishing material of high dielectricstrength, such as sand, or relatively finely dividedguartz. The embedded.elements are enclosed. in an insulatin fuse casing or housing havingterminals mounted thereon for connecting, the. fusible elements in-.parallel current .inrterrupting relation.

Upon subjection of suchaiuse unit to fault currents of a high voltagecircuit suchas produced' by a shortcircuit, the spaced apartfusibleelements, if all conditions are equal, will each attain fusintemperature and dissipateor vaporize throughout a fusible. section of.considerable length, whereupon parallel. arcing will v ccur' and themetal vapors rapidly. expand. The force of {such expansion will throwthe metal vapors out of the arc paths and into .theflspaces between thegranules of the inert 'fillermaterial separating the fusible elements.As. .a result, these. vapors rapidly condense upon thesurface of therelatively coolgranules and no longeriare available for currentconduction so that each arc is confined to the elongated and restrictedpath previously occupiedby the correspondin fusible element. Theintimate heat exchange relation betweenthe spaced apart parallelelongatedhot arc=columns and the relatively'cool granules will, if allconditions are equal, cause an exceedingly rapid transfer of heatfromeach arc column'to the adjacent granulesso that the arc energy isquickly dissipated by this heat transfer with practically"negligible netpressure on i the fusing housing; 7

,.As a result of the fusin of the parallel el'ements. in the-manneroutlined above there-willbe in effect; suddenly inserted into the pathpf-the currentat the instantof vaporization 'of thepar -allel fusibleelements a high resistanceiformed by the elongatedjparallel. arc columnswhichiwill initially limit the current to'a magnitude which is 'onl-yafraction of thecurrent which theishort circuit is otherwise capable ofproducing. IIhe inserted. resistance will cause rapid. currerit decayand the quick -O001il'lg .of. the arcs. will-pro.- .duce 'a subsequentinterruption-of :the current flow in short circuits within theinterruptinglicaiipacity of the fuse unit. But wheneversuchzinterrupting capacity is exceeded or'wheneverrall conditions arenot equal the multiple element .fuse unitmay fail to extinguish someone-'ormore of the parallel. arcs resulting from the fusing-of theparallel-elements and therefore failato :successfully interrupt theshort circuit.

As the use of. such current :limiting .powerifu'se units-becomesextended to even highermvoltage power circuits the problem :of,correspondingly increasing the voltage interrupting ability uf;=amultiple element fuse unit with-aconstruction ofeconomicsizeand=manufacturing cost becomes more-acute.

Thus, the principal object of the present invention is to provide animproved means :forincreasing the voltage: interrupting ability ofamultiple element 2 current limiting fuse unit without increasing itssize or length or -materially in- .cr.easing. its cost.

ln accordance with the present invention, this is accomplished byproviding conducting members or bands .for interconnecting :the'elongated fusible elementsin parallel current'equalizingrrelation' atspaced. intervals along. their length. Since these elements :areconnected in parallel current interrupting relation by the fuse ter=-minalsat the endsof the elements the eifect'of theadditionalintermediate conductingmembers or bands causethe currentrtodivide :more evenlyfbothfibetoreand during circuit interrupt.- ingconditions. in thesectionsof, thewfusible elements betweenthe. bandsthanit'mayinthe end sections. Such current equalization produces moreequal distribution: of .theinterruptingduW between. the elements. .Thihas been established'by'test as the successful voltagerinterruptingability ofa multiple element current .limitmg fuse has'beensubstantially improved-solely by the provision of the equalizinconducting members or bands interconnecting .the fusible elements at.spaced intervals along thirdength A further effect of the bands is tocause each current limiting fuse element to produce more voltage drop atany value of current since the voltage gradient is always higher at theends of an arc than at the intermediate portions of the arc. Thus, thebands of the present invention in effect provide additional arcingterminals intermediate the end' terminals of the multiple elements ofthe fuse. In this way the total are drop resistance of the fuse unit ismaterially increased.

But upon operation of a current limiting fuse in a highly inductivecircuit, the sudden insertion of a relatively high are drop resistanceand rapid decay of the current may result in a voltage surge which undersome conditions ma'y attainfuse-element with sections of differentfusing current capacity. Thusthefuse elements may be provided withsections of different crossflsec- 'tional area or with sections ofdifferent resistance characteristics such that sequential or stepby-.step fusing and dissipation of the fusible sections of each element issecured. In either case, when the different sections of the elements areformed to fuse in sequence, the interconnecting conducting members orbands of the present invention preferably are applied to interconnectthe elements at the junctions of the sections and preferably with thesection that will first fuse intermediate the bands so as moreeffectively to produce the joint fusing of the different sections of allelements in the desired sequence.

Further objects and advantages of the invention will become apparentfrom the following descrlption of the accompanying drawings and thefeatures of novelty which characterize the invention will be pointed outwith particularity in the appended claims.

In the drawing,

Fig. 1 is'a side view partly in section of a typical multiple elementcurrent limiting power fuse unit embodying the improvements of thepresent invention;

Fig. 2 is a cross section view of the fuse unit shown in Fig. 1 on thesectional line 2-2 with the inert arc resistance material omitted;

Fig. 3 is a chart indicating the relative increase in voltageinterrupting capacity of multiple element fuse units attained by meansof the improvements of the present invention.

A typical multiple element current limiting fuse unit construction isillustrated in Fig. 1

as comprising an enclosing tubular casing H) formed of suitableinsulating material such as fiber or glass and having the fuse terminalsII and I2 mounted on the opposite ends thereof by means of suitablesealing material I3. As shown "in Figs. 1 and 2, three symmetricallyspaced fusible element insulating and supporting cores I5, I8, and ll,of ceramic material are mounted inside the casing II! to extend substantially the full length of the casing. As illustrated, each core has fourprojecting ribs I8 so as to form an elongated square frame or reel uponwhich a plurality of fusible elements I 9 are wound in uniformlyspacedapart relationto'be separatelyembedded in the granular refractorymaterial It, as shown three fusible elements I9 individual currentlimiting'and arc extinguishing "dependent upon the maximum continuouscurrent carrying rating of the fuse. The core construction andtheWindingofthe fusible elements in spaced apart relation-thereonpreferably is such that the elements are'uniformly distributedthroughout the availablespace inside of the fuse casing Ill.

Thusyvhen the casing is filled with the finely dividdsand or other inertarc resistance material l4, each fusible element will throughout itsactivelength be embedded in intimate heat conducting relation withthismaterial and sufficientlyisolated from the adjacentelements to obtainaction of the parallel elements.

Asshown, the three cores I5, I6, and II, are mounted in symmetricallyspaced apart relation inside casing I0 by means of a spacer plate 20 ateach end and secured in position by the holding screws 2i. The multiplefusible elements 19 on each core are interconnected in parallelcurrentinlterrupting' relation at the ends thereof by mean of a commonterminal 22 which extends through suitable openings'inthe spacer plate20 and is joined as by welding to one of the fuse terminals II or I2. Inthis way, all of the elements I9 on all of the cores are connectedbetween terminals I I and I2 in parallel current interrupting relation.

In accordance with the present invention, the current equalizingconducting members Or bands 24 and 25 are provided for interconnectingall of the fusible elements IE1 at spacedintervals intermediate theirends to increase the voltage interrupting ability of the fuse unit. Asshown, the three fusible elements I!) wound on each core areinterconnected in current equalizing relation by the bands 24 and 25 atequally spaced intervals intermediate their ends and the correspondingbands on each core are then interconnected by suitable jumpers 21 so asto electrically interconnect all of the elements on all of the cores. Ifdesired, the separate bands 24 and 25 on each core may be formed simplyby winding andwelding a few turns of bare silver or zinc wire tointerconnect all the. fusible elements thereon and twisting the ends ofthe wire together and leaving these ends sufficiently long forconvenient joining as by welding with the ends of thecurrent equalizingbands on thead-w jacent cores.

When the fusible elements are provided with sections of different fusingcurrent capacityand arran ed so that the middle section will fuse beforeeither of the end sections, then the bands. and 25 are located adjacentto or overlying the junctions between the different sections of thefusible elements so as to thereby interconnect each of the correspondingsections of each of th elements in current equalizing relation. I

It has been found by test wherein the test circuit voltage is increasedthat a multiple element current limiting fuse unit provided with currentequalizing bands constructed and arranged'asdescribed above will have anincreased voltage interrupting ability over that provided by the samefuse unit without the bands. Fig. 3 has been plotted from the results ofthese tests to indicate the substantial increase in successfulinterrupting ability that was obtained solely by applying the currentequalizing bands in accordance with the present invention. As indicatedin Fig. 3, the length of a multiple element fuse unit normally requiredfor successful interruption will increase as indicated by the dottedline A as the voltage of the circuit is increased. Thus the over-alllength of the fuse unit of the same internal construction becomes afairly accurate indication of the successful voltage interruptingability thereof. The solid line B shows that when a pair of currentequalizing bands are applied to the multiple element fuse unit, asubstantial increase in successful voltage interrupting ability thereofis obtained without any increase whatever in over-all length of theunit.

While there may be various technical theories as to Why the provision ofthe current equalizing bands intermediate the ends of the multipleelement fuse in accordance with the present invention provide theimproved results indicated by Fig. 3, this can be explained mostsatisfactorily on the basis of the equalization of the current in allsections of all of the elements. Such current equalization moreeffectively insures that each section of each element will participatein and share the joint circuit interrupting duty. In this way the bandseffectively prevent a disproportionate share of the circuit interruptingduty becoming imposed upon some one element so that the arc thereofpersists and the fuse unit fails to interrupt the short circuit.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A multiple element current limiting fuse unit having in combination,a casing formed of insulatin material having arc extinguishing materialtherein and having a pair of current conducting terminals mounted on thecasing in spaced apart relation, a plurality of fusible elements inintimate heat exchange relation with the are extinguishing material andconnected in parallel current interrupting relation between theterminals, and means including a conducting member interconnecting theelements intermediate the ends thereof for equalizing the distributionof the current between the elements to increase the voltage interruptingability of the fuse unit,

2. A multiple element current limiting fuse unit having in combination,a casing formed of insulating material having arc extinguishing materialtherein and having ,a pair of current conducting terminals mounted onthe casing in spaced apart relation, a plurality of fusible elements inintimate heat exchange relation with the arc extinguishing material andconnected in parallel current interrupting relation between theterminals, and means including at least a pair of conducting membersinterconnecting the elements at substantially equally spaced intervalsintermediate the ends thereof for equalizing the distribution of thecurrent between the elements to increase the voltage interruptingability of the fuse unit.

3. A multiple element fuse unit having a plurality of fusible elementsconnected in parallel current interrupting relation and each elementhaving sections of diiferent fusing current capacity to producesequential fusing thereof, and means including a conducting memberinterconnecting the junctions between the sections of the elements forequalizin the distribution of the current to increase the voltageinterrupting ability of the fuse unit.

CLYDE G. DEWEY No references cited.

